I'm no expert on buddhism, but it seems all these buddhist countries use capital punishment. I still don't know of any countries with a government based on buddhist ideas. In any case, reincarnation isn't exactly a certain thing, so sparing a violent criminal who could be let loose in the future to kill more people on a risky bet that the person will be reincarnated as a nicer person decades later isn't exactly a good idea. Just my $0.02.